1. Field of the Invention
The present invention relates to an optical data recording medium, and more particularly to an optical data recording medium of a write-once type, which can perform recording and reproduction of data using laser light.
2. Description of the Related Art
Conventionally, a write-once optical data recording medium (optical disc) which can be recorded with data by laser light only once is called a CD-R, and is widely known. This CD-R type optical data recording medium typically has a structure in which a guide groove (pre-groove) is formed in a disc-shaped transparent substrate for tracking of laser light that irradiates during recording, and a dye recording layer composed of an organic dye, a light reflection layer made of a metal such as gold or the like and, further, a resin-like protective layer are laminated onto the substrate in that order. For recording data on this optical disc, the optical disc is irradiated with laser light in the near-infrared region (usually, laser light having a wavelength around 780 nm), irradiated parts of the dye recording layer absorb the light and locally increase in temperature, optical properties of these parts change due to physical or chemical changes (for example, formation of pits or the like), and data is recorded. On the other hand, for reproduction of data, usually, the optical disc is irradiated with laser light having the same wavelength as that of the laser light for recording, and a difference in reflectance between regions of the dye recording layer having changed optical properties (the recorded parts) and regions having unchanged optical properties (non-recorded parts) is detected.
However, CD-R is a medium whose premise is a system in which data is recorded by a user using a simple recording apparatus. Therefore, accuracy in forming pits is lower with CD-R than in the case of a ROM-type medium. Consequently, there is a problem in that irregularity in pit length, called jitter, increases and poor reproduction, for example, in the case of a decrease in reproduction accuracy of a reproduction apparatus or the like occurs.
The present inventors have intensively studied various methods for ameliorating jitter to solve the above-mentioned problem, and resultantly have found that jitter is ameliorated by decreasing the thickness of a dye recording layer. However, there is a problem in that simply decreasing the thickness of the dye recording layer reduces recording sensitivity.
The present invention has been made in view of the above-mentioned conditions, and an object of the present invention is to provide an optical data recording medium having excellent recording sensitivity and recording property (jitter).
For attaining the above-mentioned object, the optical data recording medium of the present invention has a transparent substrate in which a pre-groove having a depth of 20 to 100 nm is formed, and a dye recording layer, which is formed on the substrate and which has a thickness at a groove portion of 50 to 160 nm, has a thickness at a land portion of 80% or more of the thickness at the groove portion, and includes organic dye which enables recording of data.
In the optical data recording medium of the present invention, even if the thickness of the dye recording layer is as small as 50 to 160 nm, data can be recorded with excellent recording sensitivity and excellent jitter can be obtained, because the depth of the pre-groove formed on the transparent substrate is from 20 to 100 nm and the thickness at the land portion is 80% or more of the thickness at the groove portion, such that a difference between the thickness at the land portion and the thickness at the groove portion is small.
In the above-described optical data recording medium, the depth of the pre-groove is preferably from 30 to 90 nm, and more preferably from 40 to 80 nm. The width of the pre-groove is preferably from 400 to 630 nm, more preferably from 420 to 600 nm, and particularly preferably from 450 to 580 nm. An extinction coefficient k of the dye recording layer at a laser recording wavelength is preferably from 0.05 to 0.15, more preferably from 0.06 to 0.12, and particularly preferably from 0.07 to 0.10. The thickness of the dye recording layer at the groove portion is preferably from 70 to 150 nm, and more preferably from 80 to 140 nm. The thickness at the land portion is more preferably 90% or more of the thickness at the groove portion, and it is particularly preferable if the thickness at land portions is substantially the same as the thickness at groove portions.
The optical data recording medium of the present invention can be produced by applying an application solution having a dye concentration of from 0.5 to 1.5 g/100 ml onto the surface of the transparent substrate, at the side thereof at which the pre-groove is formed, to form a coating film, and drying the thus-formed coating film.
In the above-mentioned production method, it is preferable to use, as an application solvent, a mixed solvent prepared by mixing a main solvent, which may be a fluorine-based solvent such as 2,2,3,3-tetrafluoropropanol or the like or an alcohol solvent such as ethanol, n-propanol, isopropanol, n-butanol, diacetone alcohol or the like, with a glycol ether such as ethylene glycol monomethyl ether (methyl cellosolve), ethylene glycol monoethyl ether (ethyl cellosolve), ethylene glycol monopropyl ether, propylene glycol monomethyl ether, propylene glycol monoethyl ether, propylene glycol monopropyl ether, methylene glycol monomethyl ether, methylene glycol monoethyl ether, methylene glycol monopropyl ether and the like. By use of such a mixed solvent, a coating film having uniform thickness can be formed with the above-mentioned application solution having a dye concentration of from 0.5 to 1.5 g/100 ml.
As an application method, a spin coating method is preferable. In the case of formation of the dye recording layer by the spin coating method, the amount of the application solution fed is preferably from 0.1 to 1.0 ml per disc, and the speed of rotation during supply of the application solution is preferably from 100 to 500 rpm. After the application solution has been supplied, drying is conducted while the speed of rotation gradually increases. The speed of rotation during drying is preferably from 1500 to 8000 rpm, and the acceleration in increasing the speed of rotation is preferably in a range from 10 to 3000 rpm/s. The time required from commencement of application to termination of drying is preferably in the range from 3 to 15 seconds. If this time is too short, uniform film thickness distribution can not be obtained, and if too long, productivity decreases.
In the optical data recording medium of the present invention, a light reflection layer is preferably provided on the dye recording layer. The protective layer is preferably provided on the surface at a side of the dye recording layer which is opposite to the side at which the substrate is provided.